They're not offended when Clinton talks about God because they know very well that Clinton's an antheist just like his wife.
Because they know they don't mean it.
God talk by a conservative Republican like George W. Bush is not tolerated, whereas liberal Democrats can talk about God as much as they want, even for explicitly partisan purposes.
The reason for that is so obvious, people know that when President Bush mentions Jesus that it comes from his heart and when THEY mention Jesus it comes from their mouth. A fake believer does not intimidate them. Is there such a thing as a Democrat that really believes?